If the bullets are marring along the top when feeding, check the little flat piece of steel inside the top of the receiver just behind the chamber -- it's held in by a screw that at first glance appears to be a scope mount screw.

I had a problem that sounds similar, and I had to bend that piece of steel downward so it guided the bullet into the chamber instead of allowing it to jam into where the top of the chamber meets the receiver.

All I did was take the little flat metal piece out, hold (with pliers) the half of it without the screw hole in it, and bend it.